(1) Field of the Invention
This invention relates to a rotary encoder capable of measuring, as a digital absolute magnitude, the number of revolutions of an operation-controlling rotary shaft in an automatic control apparatus, robot apparatus, manipulator apparatus or the like, a power-transmitting rotary shaft used for feeding a material or for opening and closing a valve or the like.
(2) Description of the Prior Art
Increment-type (or relative-type) instruments and absolute-type instruments have heretofore been employed to measure digitally the number of revolutions of a controlling or power-transmitting rotary shaft. The term "number of revolutions" as used herein may also be called "the driving number" but will hereinafter be used to mean the number of actual revolutions in order to avoid its confusion with the number of revolutions per unit time.
Increment-type instruments have simple structures and can thus be used economically. They have hence found wide-spread commercial utility in various control apparatus and systems.
However, increment-type instruments are accompanied by such drawbacks that they require initial presetting such as adjustment to original points, and because necessary data are lost when their power sources are cut off, automatic control systems are actuated erroneously and may induce accidents even when their power sources are momentarily cut off.
Among absolute-type instruments, there are potentiometers, encoders making use of encoder plates, gear devices, etc.
Absolute-type instruments relying upon potentiometers are of the analog system. In order to obtain digital signals, it is necessary to subject each measurement result to an analog/digital conversion. Besides, they are susceptible to influence such as drift. In addition, the numbers of revolutions which may be input to such instruments are limited to about 10 revolutions or so. Unless reduced by means of a train of gears, such potentiometer-relying instruments cannot be used to measure relatively-higher numbers of revolutions.
On the other hand, absolute-type instruments each of which is equipped with an encoder, which is in turn constructed of a single piece of encoder plate, so as to obtain many bits can be applied only where the number of revolutions to be input is one revolution or less. Their resolutions are also limited. Therefore, it is not likely to obtain any high-digit output from such instruments.
As instruments capable of solving the above-mentioned drawbacks, there have been proposed instruments each of which makes combined use of a train of gears and an encoder employing an encoder plate capable of obtaining many bits at the same time.
In each of such newly-proposed instruments, the number of input revolutions is successively reduced at constant ratios by means of a train of gears. With respect to each stage of the thus-reduced train of gears, the angle of each rotation is encoded by means of an encoder which is able to obtain a plurality of bits.
In the above case, the resolutions of encoders which resolutions are required for the respective stages vary depending on their respective reduction ratios relative to the number of input revolutions.
Let's now assume by way of example that the reduction ratio per each stage be 1/10. Where a train of gears consists of three stages meshed with one another, the angles of rotations are respectively 36 degree in the first stage, 3.6 degree in the second stage and 0.36 degree in the third stage. Therefore, such instruments are accompanied by a drawback that an extremely high level of accuracy is required for the highest stages.
Even if a high level of accuracy should be achieved for the highest encoder, a train of gears having usual machining accuracy cannot avoid errors which occur when the direction of revolutions is reversed, for example, due to backlash or the like.
The adverse effect of such backlash is directly conveyed to the highest stage. The backlash is accumulated as gears are meshed in a higher stage, and the thus-accumulated backlash is reflected to the highest digit. Therefore, even if the accuracy of each encoder is improved, the upper value of practically-countable revolutions is limited by mechanical inaccuracy such as backlash and the like so long as a train of gears is employed.
The adverse effect of backlash arises as a hysteresis phenomenon of each rotary encoder when the direction of its revolution has been reversed. If such an adverse effect occurs either before or after a carry, it will come out more seriously as an error in the upper digit. This is another drawback of the above-proposed instruments.
Moreover, mechanical inaccuracy such as backlash and the like increases by wearing, abrasion or the like of the train of gears. Therefore, it is infeasible to expect stability and reliability over a long period of time.